As one example of the display modes of a liquid crystal display device, there is a vertical alignment mode wherein the liquid crystal molecules in the liquid crystal cell are aligned vertically with respect to the substrates thereof under the initial conditions. When no electric voltage is applied, the liquid crystal molecules are aligned vertically with respect to the substrates. Therefore, a black image is obtained if linear polarizers are arranged on the both sides of the liquid crystal cell perpendicularly to one another.
The optical property in the liquid crystal cell is isotropic in the plane direction, and thus ideal viewing angle compensation is easily achieved. In order to compensate a positive uniaxial anisotropy in the liquid crystal cell thickness direction, an optical element with a negative uniaxial anisotropy in the thickness direction is inserted between one or both surface of the liquid crystal cell and the linear polarizers, resulting in extremely excellent black display viewing angle characteristics.
When an electric voltage is applied, the liquid crystal molecules changes in alignment from the direction vertical to the substrate surfaces toward the direction parallel thereto. Thereupon, it is difficult to align uniformly the liquid crystal molecules. The use of a conventional alignment treatment, i.e., rubbing treatment leads to a significant deterioration in display quality.
In order to align uniformly the liquid crystal molecules when an electric voltage is applied, a proposal was made wherein the shapes of electrodes on the substrate are modified to generate an oblique electric field in the liquid crystal layer. According to this proposal, although a uniform liquid crystal molecule alignment is achieved, nonuniformly aligned regions if viewed microscopically are formed and become dark regions when an electric voltage is applied. Therefore, the liquid crystal display device is decreased in transmissivity.
According to Patent Document 1 below, a proposal was made wherein circular polarizers are replaced for linear polarizers arranged on both sides of an liquid crystal element having a random-aligned liquid crystal layer. The use of circular polarizers each comprising a linear polarizer combined with a ¼ wavelength plate in place of the linear polarizers can eliminate dark regions formed when an electric voltage is applied and accomplish the production of a liquid crystal display device with a higher transmissivity. However, a vertical alignment type liquid crystal display device with circular polarizers has a problem that the viewing angle characteristics are narrower than a vertical alignment type liquid crystal display device with linear polarizers. According to Patent Document 2 below, it is proposed to use an optically anisotropic element with a negative uniaxial anisotropy or a biaxial optically anisotropic material in order to compensate the viewing angle of the vertical alignment type liquid crystal display device with circular polarizers. However, the optically anisotropic element with a negative uniaxial anisotropy can compensate the positive uniaxial optical anisotropy in the liquid crystal cell thickness direction but can not compensate the viewing angle characteristics of a ¼ wavelength plate, resulting in a failure to obtain sufficient viewing angle characteristics. Further, upon production of the biaxial optically anisotropic material, NZ defined as Nz=(nx−nz)/(nx−ny) is −1.0<Nz<0.1 wherein nx and ny indicate the main refractive indices in the plane of the resulting optically anisotropic plate, nz indicate the refractive index in the thickness direction, and nx>ny. Therefore, there is a limit in stretching in the thickness direction and thus the retardation in the thickness direction cannot be controlled within a wide range. Further, since in the foregoing production method, an elongate film is stretched in the thickness direction by utilizing the heat-contraction of a heat contractive film, the resulting retardation plate becomes thicker than the original elongate film. The thickness of the retardation film produced by the method is from 50 to 100 μm and was insufficient for low profiling required in liquid crystal display devices or the like.
In order to compensate the viewing angle of a vertical alignment type liquid crystal display device with circular polarizers, Patent Documents 3 and 4 below proposes a structure wherein three types of elements such as an optically anisotropic element with a negative uniaxial optical anisotropy for compensating the liquid crystal cell, a compensation layer that is large in refractive index in the thickness direction for compensating the viewing angle of the ¼ wavelength plate and a polarizer compensating film are combined. However, when each of these 3 types of the films are arranged on both sides of the display device, 6 sheets of the films in total are used and further λ/4 plates are used on both sides for imparting a circular polarizer function, resulting in the use of 8 sheets of these films. Therefore, they can significantly improve the viewing angle but are not practical in view of production cost and thickness.